Activated protein C (APC) is a naturally occurring plasma serine protease that has been translated to the clinic as a recombinant wild type or mutant biologic. In a diverse collection of preclinical animal injury models, pharmacologic APC provides benefits. APC not only has anticoagulant activity but also initiates cell signaling via multiple receptors, in particular via several protease activated receptors (PAR). APC-initiated cell signaling contributes to tissue homeostasis and host defense systems. Beneficial APC-initiated biased signaling is caused by specific cleavages of PAR1 and PAR3, and it also can be triggered by APC binding to Tie2 on endothelial cells. Despite recent insights, there is a major gap in knowledge about protein-protein interactions (PPI) between APC and its cellular receptors. Aim 1 studies will use a library of 28 recombinant APC mutants to provide a database regarding APC's receptor specificities which will then enable engineering of APC mutants with receptor-specific selectivity, e.g., an APC mutant with highly selective PAR1-specific or PAR3-specific signaling capabilities. Such receptor-selective APC mutants will be useful reagents for deciphering which receptors play critical roles on cells in vitro or in animals in vivo, and they may lead to translation for novel APC mutants. One major anti-inflammatory mechanism for APC is its recently discovered ability to inhibit NLRP3 inflammasome activation. There a major need for understanding how APC inhibits inflammasome activation, and Aim 2 studies will provide highly novel new knowledge. When APC is generated in excess relative to thrombin generation, increased risk for bleeding arises. This may potentially occur in hemophilia or during use of direct oral anticoagulant (DOAC). Aim 3 studies will provide new knowledge about bleeding and joint damage in murine hemophilia models linked to relatively excessive APC and will determine whether various strategies may reduce joint damage that arises due to bleeding in hemophilic joints. The proposed studies will provide novel mechanistic insights and new APC variants which may aid translation related to the APC pathways.